Patients suffering from certain ailments, including end-stage renal disease (ESRD), may undergo hemodialysis treatment. Hemodialysis is a process in which arterial, or outward flowing, blood is removed from the body, filtered, and subsequently reintroduced as venous, or returning, blood. In order for the dialysis process to be efficiently carried out, a relatively high blood flow rate is required (approximately 600 ml/min).
While the vascular system is capable of providing these flow rates, safely accessing the vascular system can be problematic. For example, while a central venous catheter (CVC) may provide immediate access to the vascular system close to the heart at sufficiently high flow rates, it is associated with a high risk of infection due to permanent exposure to external environmental conditions through the entrance site. Accordingly, the use of a CVC is not a viable option for long term access to the vascular system for hemodialysis purposes.
Directly accessing, or cannulating, the artery is similarly not viable, as it generally requires inserting two needles directly into the artery (piercing the arterial wall) for each dialysis treatment. As patients may undergo dialysis 3-4 times a week, the repeated perforation of the arterial wall may ultimately lead to destruction of the artery through stenosis. Moreover, accessing the artery itself may cause heightened pain and discomfort to the patient. Cannulation of the artery may also be practically limited, in that arteries with sufficiently high flow rates are harder to find within the body and more difficult to access.
In order to facilitate repeated access, as required for treatment of ESRD by hemodialysis, surgeons have historically resorted to grafts to create a bridge between an artery and a vein, where the blood flowing in the artery may be diverted, in part, to flow through the graft and into the vein. The process effectively bypasses narrower, more distally located, capillaries to achieve the desired flow rates. Hemodialysis may then be performed through cannulation of the graft, as distinct from the artery itself.
While repeatedly accessing the graft is preferable to cannulating the artery, the graft too may deteriorate over time and may require subsequent intervention to repair or replace the graft. The material used to form the graft (e.g., ePTFE tubing), may also increase the likelihood of forming occlusions, or clots, on the inner lumen of the graft, as it places a foreign material (e.g., the entire length of ePTFE tubing, e.g., 30-45 mm) into contact with the blood stream. In this way, the graft may narrow over time and may be rendered unsuitable for use in hemodialysis, requiring further intervention (e.g., a balloon dilation or an additional graft).
The use of grafts is generally not a preferred method for vascular access, and surgeons often favor the surgical creation of an arteriovenous fistula. At a high level, the surgical process aims at attaching a vein directly to an artery, bypassing the distal capillaries to achieve the necessary flow rate. The surgical procedure typically begins by exposing the desired vessels, identifying and ligating a target vein and artery, to prevent blood flow into the surgical site. The vein, having a distal portion in the patients hand and a proximal portion towards the patient's heart, is completely transected, which may introduce stress due to the vasospasm of the transected vessel. The distal end of the proximal portion is secured prior to transection and is then bent towards the artery. The distal end may be splayed open slightly, and attached around a longitudinal incision made in the artery, thereby allowing for blood to flow from the artery into the vein once the artery is no longer ligated. The surgical procedure itself is quite delicate, and may be difficult to perform for even the most experienced surgeon.
In order to successfully form a fistula, the vein once transected and attached to the artery must undergo a maturation process in which the vein enlarges, or dilates, in order to accommodate the increased pressure and higher flow rates. While the fistula maturation process is poorly understood, the maturation process is driven by a positive feedback mechanism in which high flow rates and increased pressure lead to dilation of the vein, which in turn accommodates increased flow rates and pressure. As a specific example of the fistula maturation process, which may take 2 months or more, a 2.5 mm-3 mm vein having a flow rate of <30 ml/min may mature into a 6 mm vein, providing a flow rate of roughly 600 ml/min.
Once the fistula has formed, it may be cannulated, or accessed, for each hemodialysis treatment that is performed. While the fistula may ultimately deteriorate, the risk of stenosis is relatively lower, in part, because no foreign material (e.g., ePTFE tubing) is introduced into the system. Furthermore, clinical research has shown that patients with AVFs have better long-term patency rates and reduced incidence of required intervention relative to alternative methods. Research shows that AVFs have the longest survival, result in fewer access related procedures, fewer hospitalizations due to infection and lower overall costs of care.
Even so, the surgical creation and maturation of a fistula is only successful in roughly 50-80% of patients. While the reasons for failure are not clearly understood, the physical stress and technical difficulty of the surgery may play a significant role in its failure. The failure may, for example, be associated with vascular trauma or occlusion of blood flow during surgery, which may result in neointimal hyperplasia and thrombosis. As another example, tissue downstream of the anastomosis site that proliferates to heal may over proliferate, resulting in neointimal thickening. The procedure may also fail due to stenosis, which may be associated, in part, with the cutting of surrounding muscle and tissue to mobilize the vein or the manipulation of the vein by splaying its end for attachment to the artery. Moreover, because the vein often times may move more than 10 mm in the lateral direction, it may cause bending, and possible kinking, of the vein, which may result in a phenomenon referred to as “swing stenosis.”
Also, as noted above, successful fistula maturation requires sufficient post-operative flow rates to promote vein dilation, but this may be impeded by trauma to the vein or artery or poor and inconsistent surgical technique. A primary cause of difficulty with the procedure is the vasospasm of the vein in response to ligation, which makes it much more difficult to correctly suture the vein to the artery. Additionally, while high post-operative flow rates support fistula maturation and vein dilation, they also introduce hemodynamic stresses, including the shear stress resulting from a turbulent flow and higher flow rate.